Backing 1980, professor Krashen introduces a learning second language theory. The professor Krashen talks about five-type theory of second language, including the Acquisition-Learning hypothesis, the Monitor hypothesis, the Natural Order hypothesis, the Input hypothesis, and the Affective Filter

Stephen Krashen's five Hypotheses: 1) The acquisition-learning distinction states that adults have two distinct and independent ways of developing competence in a second language: acquisition and learning.